Powers Via Possession

A Stock Super Powers trope. The character gets superpowers via allowing themselves to be possessed or via channeling spirits (usually ghosts) through them, via which they can access the skills and abilities the ghosts had in life or new powers that the ghost possesses via dint of, well, being a ghost such as unearthly wailing, freezing touch, moving things around like a poltergeist.

Of course, this requires the ability to see and talk to the ghosts in the first place.

May end up with the character possessing such powers referred to as being a medium, channeler, shaman, or witch doctor.

Examples of Powers Via Possession include:

Anime and Manga

The core premise of Shaman King - although the protagonists are only possessed by their spirit partners at the beginning. Once the tournament actually starts, it becomes clear that just about every shaman worth fighting merges their spirit with one of their weapons.
One Piece has Luffy—at one point, force fed the shadows of 100 fallen pirates, giving him all of their skills and strengths for 10 minutes. Other than now being skilled with a sword, mostly this just causes Luffy to be much bigger and stronger than usual.
Kurau and other people possessed by Rynax in Kurau Phantom Memory gain numerous Rynax powers such as Flight, intangibility, and the ability to disintegrate things.
In the Kurosagi Corpse Delivery Service, Yata claims to be a channeler who channels the mind of an alien into an old sock puppet he wears. Much of Kuro's itako powers comes from being possessed by a ghost. The ghost never interacts with him directly -- it takes several issues before he even becomes aware of her existence.
A member of the Phantom Brigade in Hunter X Hunter can control people if he sticks them with a cell phone antenna. If he uses this on himself, he becomes faster, stronger, and more resistant to pain, because his self-possession disconnects his mind from the sensations his body feels, allowing him to surpass his body's physical limits. He doesn't like using it, though, because he will feel all the pain his body went through once the possession ends.
Several characters on Tokko have "phantoms" living inside them, granting them powers such has enhanced strength and speed, Healing factors, and in some cases, Sword Beam attacks.
Devilman kicks off with the main character Akira becoming possessed by Amon, the most powerful and evil demon in Hell. Amon's power allows Akira to become the titular Devilman. As long as Akira can hold onto his ability to think rationally, Amon cannot take control of him, though Akira does become stronger and more aggressive after his possession. When Akira loses his reason after his girlfriend is violently killed by an angry mob, Amon takes control of him and this becomes straight up Demonic Possession.
In Fairy Tail, Mirajane, Elfman, and Lisanna use Take Over magic, which works this way but with monster souls. Elfman uses beast souls, Lisanna uses animal souls, and Mirajane uses Demon souls.

Comic Books

In the comic Haunt, The main character is possessed by the spirit of his dead brother, and gains super powers.
In PS238, one of the children is a medium who can see ghosts and allow them to possess her to gain their knowledge. If the person had superpowers in life, she can get access to those as well.
Marvel Comics' Captain Universe is all about this trope.
This is Deadman's stock in trade in The DCU. Except he does the possessing.
In the versions of the story that make Jason Blood and Etrigan separate entities, the only "benefit" that Jason Blood gets from being Etrigan's host is sharing the demon's immortality. Any powers that he possesses as Jason are separate from Etrigan's and are the result of having centuries to improve his magical abilities.
The Entities in The DCU grant their hosts massive power related to the emotions they embody. Granted, with the more Obviously Evil Entities like Parallax, it's more akin to Demonic Possession since the host usually has little choice in the matter and the Entity is in the driver's seat. The only straight example of this seen so far is Ion the Willpower Entity—being respectful of the free will of others, it does not influence its hosts' thoughts or actions in any way.
- It's unclear how much control some of the others have. It's been suggested that the Predator's (Love Entity) status as Stalking Is Love Complete Monster is because its attracted to "broken" hosts (since the Sapphires see their role as being to "fix" love), but then accepts the host's own severely flawed interpretation of what love is. Nicole Morrison, the host of the Hope Entity Adara, became calm, wise, and all-forgiving, but still seemed to be basically Nicole Morrison.

Fan Works

In the X Men fic The Wraith Saga, Jason "Mastermind" Wyngarde gets an upgrade from Master of Illusion to full-blown Reality Warper after he's possessed by the Phoenix Force's Evil Counterpart, a shadowy Eldritch Abomination called "The Wraith".
In many of the stories of Fred Herriott will be found the Nendo-kata, intelligent octopus-like aliens with massive psionic powers who can "merge" with members of other sapient species. The merger is not as metaphysical as other examples on this page and leaves behind the corpses of both participants, who literally combine their souls to inhabit a new body. In all the cases he's written about, this new body (and the personality within it) has been essentially that of the human participant, with some changes -- and the massive psionic powers.

Film

This is the premise of how the young girls become witches in The Craft. They already have some magical ability, but then try to amp things up by invoking some guy called "Manon" to possess them. The god in Wicca is not called Manon and this was made up for the film. In fact almost the whole film was made up, to the great annoyance of Wiccans in the real world.

Literature

Mortimer Lindquist reveals in Ghost Story that he has the ability to borrow the skillsets and abilities of ghosts near him and use them for his own means. Including Harry's basic magical abilities, when he calls up a simple defensive spell.
- Harry does the same for Molly when she's surrounded by Fomor henchmen. Works especially well because Molly had been fighting them using illusions, but Harry fights with fire. After general badassery and crispy henchmen ensue, it turns sad: Molly has always crushed on Harry, and she observes that it's the closest she'll come to getting with him.
Kalinda in Star Trek: New Frontier has the ability to talk to and channel ghosts. In Treason, her dead brother Si Cwan possessess her, aiding in the Excalibur's search for his and Robin's newborn son. While possessed, she gains all of his fighting prowess and intellect, while Squicking Robin out (and Kalinda's other lover, Xyon). About the only person Cwan!Kally doesn't squick out is Tania Tobias, who's a little touched herself...and strikes up a relationship with Kalinda that lasts after the possession ends, to Xyon's utter dismay.
This is how all sorcerers and saints gain their powers in Lois McMaster Bujold's Chalion novels; possession by a demon makes one a sorcerer, while possession by a god makes one a saint. Unfortunately, both will make you Blessed with Suck, since demonic possession usually leads to a Split Personality Takeover, whereas possession by a god makes you a Cosmic Plaything.
After being briefly possessed by F'ryan (A swordmaster) to fight off a Weapon, Karigan retains some of his skills. Later volumes in the Green Rider series have her build on that base with more conventional training.

Live Action TV

Angel has Illyria, an ancient god in a human body.
- In season 4, Cordelia is granted magics powerful enough to make her stand against Willow Rosenberg in a magical duel, due to her mystical possession by and pregnancy with Jasmine.
Kamen Rider Den-O functions under this with the Imagin. While Ryotaro can access Den-O's powers without being possessed, he can only use most of them in tandem with the Imagin. Possession was especially useful early on because he was the definition of Non-Action Guy and was more likely to hide behind something or faint than engage in chop-socky when faced with monsters.
- Lampshaded sometimes when the Imagin leave Ryotaro at some of the most inoppurtune moments, resulting in him either running for his life or finding himself in someone else's house.
  - Especially in Ryutaros' case whom has no sense of self control.

Myths & Religion

Voodoo (Vaudon) magic is supposed to work this way, as the faithful are "ridden" by the loa they summon.
In both traditional and modern Spiritualism, voluntary possession (partial or full trance mediumship) allows spirits to continue doing things in the present world. Healer mediums invite doctors and traditional healers in. In the early days of modern Spiritualism, Thomas Jefferson, Ben Franklin and others used to give abolitionist speeches this way.

Tabletop Games

The main premise of Geist: The Sin Eaters.
- It's a little more complicated than that.
The Dungeons & Dragons supplement Tome of Magic introduced Binders, practitioners of "Pact Magic". These characters, though not spellcasters themselves, would conjure up entities called Vestiges and let a fragment of the things' consciousness "tag along" in exchange for supernatural abilities. Binders usually manifest tell-tale mutations or certain behavioral compulsions afterwards, which vary depending on which Vestige is involved.
- Fourth Edition, in addition to having a variation of Binder, also applies this to Barbarians, whose main gimmick is using nature spirits to go into raging trances.
In Warhammer 40,000, some Chaos Space Marines voluntarily undergo Demonic Possession in exchange for power, while some of the more radical Inquisitors create Daemonhosts by binding a daemon inside a poor sap's body.
- Daemonic Possesion MAY sounds cool (the benefits aren't so bad), but it also involve an unhealty dose of Body Horror.
- And, of course, the entire your-puny-human-mind-is-shredded-by-the-sheer-power-of-Chaos thing.

Video Games

Variation in Phantom Brave: you get the ghosts to posses objects to access their skills, but the premise is close enough.
Used to assist courtroom cases in the Ace Attorney series.
Shrine Maidens like Reimu from the Touhou series are said to be able to do this, though it's only been seen in Silent Sinner in Blue, and with gods rather than spirits.
Zappa from the Guilty Gear games probably wouldn't be anything close to a match for the other fighters if it wasn't for the fact that he's possessed by a very angry ghost. Or ghosts, depending on your interpretation.
City of Heroes' "Kheldian" characters get their powers from a an alien energy-being inhabiting their body.
The Collector General actually (just the channel for) a Reaper, Harbinger in Mass Effect 2 will often take control of one of his flunkies, transforming it from a mild inconvenience into a burning, supercharged biotic monster who will constantly advance to blast you in the face. Worst of all, it doesn't seem to take much out of him. Kill one avatar and he'll possess any surviving Collector in short order.
- ASSUMING DIRECT CONTROL.
- Also happened earlier, in Mass Effect 1: You kill Saren, either by making him shoot himself in the head, or shooting him to a pulp yourself. That should stop Sovereign/Nazara from using him any longer, right? Wrong. Bastard gets right back up, the cybernetics in him taking over the body (which might not have been quite entirely dead, judging by the screaming) and gives you another round of hell.
The main character of Soul Nomad and The World Eaters gains powers from being 'possessed' by Gig. Fortunately for everyone involved, Gig has no real control over the protagonist (which in return limits the amount of power the protagonist gains from the deal) and can, at worst, only be sarcastic at everybody.
Riku in Kingdom Hearts. First time you fight him (which is optional), he's Badass Normal and is mostly hard because he keeps jumping around and when you knock him down, flings back at you and it hurts. The second time you fight him (which is not optional), he is using the powers of darkness and puts up a fight before going down but isn't a hard opponent...partly because it's 3v1 in your favour. The third time you fight him, however, he becomes That One Boss because he is possessed by Xehanort's heartless. And it's not just because it's a Duel Boss...
- Averted in the later games; however - he later learns to use these powers without being controlled.

Web Comics

Used in Order of the Stick story arc where Vaarsuvius makes a Deal with the Devil to get "ultimate arcane power". This is accomplished by splicing the souls of three deceased epic wizards to his own, giving him access to their full selection of spells for as long as he can maintain the necessary concentration.
The Cobra Days comic (about Metal Gear Solid 3's Cobra unit) expanded Sorrow's medium power into this: getting a Ninja to possess him to take out an enemy unit.
Joel in Concession has spiritual powers greater than most who have them due to his sister's spirit, though which one is more in control is debatable.
- The same now for Artie, who is chanelling the spirit of his dead girlfriend.
Nodwick once was overrun by three intelligent weapons at once. Great carnage ensues.
Gunnerkrigg Court had Jack Hyland, a student good enough at things like bypassing security measures. Possession by an imaginary spider quickly made him a Mad Scientist with understanding of "ethereal sciences" comparable to adult specialists in this area. And gave him stock powers growing from ethereal vision to Flight. On the down side, he acted like on heavy psychostimulants - excitable, nasty mood whiplashes, no sleep - and thus after a few months of this "fun" barely managed to survive and not go nuts permanently. After he got rid of the spider, "great ideas" are gone, but he's still rather brilliant on his own and this misadventure seems to either give or awaken at least some magical potential.
The protagonist of Fusion gained superpowers related to atomic fusion after becoming the host of a rather snarky alien entity. She calls him "Bob".
In Chasing the Sunset Leaf accidentally became a host to a few spirits of wrath, who eventually agreed to let him stay in control, but remained somewhat... overeager.

Myhrad: So Leaf has Furies and Ayne has this Lord. Seems I'm the one with the least dangerous relations, and I'm a dragon.

Web Original

Ghostkeeper, a Canadian hero active in the Global Guardians PBEM Universe gains the accumulated strengths and skills of his ancestors by letting their spirits possess him. Maahes and Minhit are a pair of Egyptian supervillains who both get their power from an ancient lion-spirit of bloodshed.
- Uncle Sam, from the same setting, is the incarnated Spirit of America.
Seen in the Whateley Universe; the mutant "avatar" trait is all about being able to host a spirit in the mutant's body and derive power from that union. It's not even all that uncommon—the school has quite a few examples, and one of the world's premier heroes is the host of the so-called "Champion Force", an amalgamation of multiple spirits collected by a predecessor.
The Nostalgia Chick's best friend Nella gets these when she's taken over by a quasi-demonic, intergalactic force of evil and becomes Dark Nella.

Western Animation

Avatars can do this in Avatar: The Last Airbender. Each Avatar is born with the spirit of all of the previous Avatars within them. The Avatar can learn to summon those spirits into himself, and thus gain the power of thousands of previous benders. Additionally, Aang was once possessed by the spirit of the ocean itself, allowing him to transform into a giant monster made of seawater.